Manure Manager

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Manure minute: Calibrate your manure spreader

April 11, 2022  by Chryseis Modderman | University of Minnesota Extension

Whether you apply liquid or solid manure, applicator calibration is an important step that is often overlooked. I know you’re busy, but just taking a moment to adjust your speed and settings will improve your confidence that you haven’t over- or under-applied nutrients. Eyeballing it isn’t going to work. A study from Michigan State University found that only a quarter of farmers who didn’t calibrate were within 20 percent of their desired application rate. If you’ve already done the work of soil sampling, manure nutrient testing, and calculating application rate, why wouldn’t you calibrate your spreader to ensure you’re actually applying the amount you had planned? You should calibrate every couple of years or whenever manure consistency or equipment has changed. Note that the application methods described here are for broadcast applications of solid and liquid manure; manure injectors tend to rely on flow meters for accuracy.

How to calibrate
Calibration is actually a pretty simple process. The easiest method that works for both solid and liquid manures is to weigh the spreader before and after application to a small area. Measure the length and width of the area to which manure was spread.

If you don’t have access to drive-on scales, the tarp method can be used for solid manure. You’ll need a scale, five-gallon bucket, and a tarp. First, weigh the empty bucket and tarp. Then, lay the tarp in the field with bricks or rocks to hold down the edges. Measure the length and width of the tarp to get spread area. Next, apply manure normally to the area with three passes (one directly over tarp, the other two on either side of the tarp) to account for overlap. Then, fold up the tarp and place in bucket. Weigh the bucket and tarp full of manure.

Math time! For both the weight and tarp method, you just need to extrapolate the amount spread over the small area/tarp to amount per acre. To do this, you first need to figure out how much manure was spread over the small area/tarp. Subtract the after-application spreader weight or empty bucket/tarp weight (in pounds) from the full spreader or full bucket/tarp weight to get pounds of manure spread. Then, divide that number by the area spread/tarp area (in square ft.). Multiply by 21.8 to get tons of manure applied per acre. Note that 21.8 is derived from 43,560 sq ft per acre divided by 2000 lbs per ton – a different number will need to be used for metric units. For solid manure, that is all you need to do.


For liquid manure, since it is applied in gallons per acre, not tons, you’ll need to do an extra step. To convert tons per acre to gallons per acre, you’ll need to find the density of your manure – yes, it’s different from the density of water. This is also fairly simple. Just weigh an empty five-gallon bucket (or any container of known volume); and then weigh it again full of manure. Subtract those numbers to find how much manure occupies a five-gallon space. Divide five gallons by the manure weight in pounds, then multiply by 2000 to get gallons per ton. Finally, multiply that number by the tons per acre you calculated earlier to get gallons per acre.

That’s all there is. From here, you can decide to adjust speed or equipment settings to spread more or less per acre. It is a good idea to repeat the calibration process a few times to get a good idea of application rate. Make sure to record your settings for future reference. Happy spreading! •


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